Contents

Ketone bodies can also be used for energy. Ketone bodies are transported from the liver to other tissues, where acetoacetate and beta-hydroxybutyrate can be reconverted to acetyl-CoA to produce energy.

The heart gets much of its energy from ketone bodies, although it also uses a lot of fatty acids.

The brain gets its energy from ketone bodies when insufficient glucose is available (e.g. when fasting). In the event of low blood glucose, most other tissues have additional energy sources besides ketone bodies (such as fatty acids) but the brain does not. After the diet has been changed to lower blood glucose for 3 days, the brain gets 30% of its energy from ketone bodies. After 40 days, this goes up to 70% (during the initial stages the brain does not burn ketones, since they are an important substrate for lipid synthesis in the brain). The brain retains some need for glucose, because ketone bodies can be broken down for energy only in the mitochondria, and brain cells' long thin axons are too far from mitochondria.

Acetone is formed from spontaneous decarboxylation of acetoacetate. In a corresponding manner, the levels of acetone are much lower than those of the other two types of ketone bodies. And, unlike the other two, acetone cannot be converted back to acetyl-CoA, so it is excreted in the urine and exhaled (it can be exhaled because it has a high vapor pressure and thus evaporates easily). The exhalation of acetone is responsible for the characteristic "fruity" odour of the breath of persons in ketotic states.